1. Field of the Invention
The field of art to which this invention pertains is the solid bed molecular sieve separation of fatty acids. More specifically, the invention relates to a process for separating a fatty acid for a rosin acid which process employs a molecular sieve comprising silicalite.
2. Description of the Prior Art
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon types from mixtures thereof. As a few examples, a separation process disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491 uses a type A zeolite to separate normal paraffins from branched chain paraffins, and processes described in U.S. Pat. Nos. 3,265,750 and 3,510,423 use type X or type Y zeolites to separate olefinic hydrocarbons from paraffinic hydrocarbons. In addition to their use in processes for separating hydrocarbon types, X and Y zeolites have been employed in processes to separate individual hydrocarbon isomers. As a few examples, adsorbents comprising X and Y zeolites are used in the process described in U.S. Pat. No. 3,114,782 to separate alkyl-trisubstituted benzene isomers; in the process described in U.S. Pat. No. 3,864,416 to separate alkyl-tetrasubstituted monocyclic aromatic isomers; and in the process described in U.S. Pat. No. 3,668,267 to separate specific alkyl-substituted naphthalenes. Because of the commercial importance of para-xylene, perhaps the more well-known and extensively used hydrocarbon isomer separation processes are those for separating para-xylene from a mixture of C.sub.8 aromatics. In processes described in U.S. Pat. Nos. 3,558,730; 3,558,732; 3,626,020; 3,663,638; and 3,734,974, for example, adsorbents comprising particular zeolites are used to separate para-xylene from feed mixtures comprising para-xylene and at least one other xylene isomer by selectively adsorbing para-xylene over the other xylene isomers.
In contrast, this invention relates to the separation of non-hydrocarbons and more specifically to the separation of fatty acids. Substantial uses of fatty acids are in the plasticizer and surface active agent fields. Derivatives of fatty acids are of value in compounding lubricating oil, as a lubricant for the textile and molding trade, in special lacquers, as a water-proofing agent, in the cosmetic and pharmaceutical fields, and in biodegradable detergents.
It is known from U.S. Pat. No. 4,048,205 to use type X and type Y zeolites for the separation of unsaturated from saturated esters of fatty acids. The type X and type Y zeolites, however, will not separate the esters of rosin acids found in tall oil from the esters of fatty acids nor the free acids, apparently because the pore sizes of those zeolites (over 7 angstroms) are large enough to accommodate and retain the relatively large diameter molecules of esters of rosin acids as well as the smaller diameter molecules of esters of fatty acids (as well as the respective free acids). Type A zeolite, on the other hand, has a pore size (about 5 angstroms) which is unable to accommodate either of the above type esters (or free acids) and is, therefore, unable to separate them. An additional problem when a zeolite is used to separate free acids is the reactivity between the zeolite and free acids.
We have discovered that silicalite, a non-zeolitic hydrophobic crystalline silica molecular sieve, is uniquely suitable for the separation process of this invention in that it exhibits acceptance for a fatty acid with respect to a rosin acid, particularly when used with a specific displacement fluid, and does not exhibit reactivity with the free acids.